Currently no diagnostic imaging method offers sufficient specificity to reliably differentiate between benign and malignant breast lesions in patients. Pathologic evaluation is presently the only way to obtain a definitive diagnosis. This research will use new methods in magnetic resonance spectroscopy (MRS) and a very high magnetic field (4 Tesla) to distinguish malignant from benign breast lesions. The required specificity will be gained from biochemical properties measured non- invasively with proton (1H) MRS. This work will further investigate the earlier finding that the presence of choline- containing compounds (Cho) at 3.2 ppm in 1H spectra may be a marker for malignancy in breast lesions. It is hypothesized that false-positive diagnoses can result from a resonance at 3.2 ppm that does not arise from Cho, but instead originates from lipids or other macromolecules with reduced mobility and short transverse relaxation time (T2). A macromolecular resonance at 3.2 ppm has been shown to occur in 1H spectra of brain. In the present project, the measured difference in intrinsic T2 should permit macromolecular and lipid resonances at 3.2 ppm to be distinguished from free Cho, thus eliminating false-positives. The improved SNR offered by the proposed MRS methods and the 4 Tesla magnetic field will significantly reduce the incidence of false-negative diagnoses. As an alternative means to characterize breast lesions, the ability of MRS to measure glucose metabolism in breast lesions will also be investigated, by detecting 13C-labeled lactate following an intravenous injection of glucose enriched with the stable 13C isotope. Breast lesions will be visualized by dynamic contrast-enhanced two-dimensional MRI and high resolution three-dimensional MRI. MRI and MRS scans will be performed on clinically identified suspicious breast lesions prior to biopsy or surgical removal. Correlation with pathologic findings will reveal whether the proposed MRS measurements provide the additional specificity needed to accurately identify and characterize malignant lesions in breast patients.